Gas phase filtration has traditionally been accomplished by low, medium and high efficiency pleatable composite filter media which include either a low, medium or high efficiency fibrous filtration layer of randomly oriented fibers; and one or more permeable stiffening layers which enable the composite filter media to be pleated and to sustain its shape. Such filtration devices serve as vehicle passenger compartment air filters, high performance engine air filters and engine oil filters. ASHRAE (American Society of Heating Refrigeration and Air Conditioning Engineers) pleatable filters and the like typically use a pleated high efficiency filtration media for the filtration element.
Currently, the pleated high efficiency media normally used in these filtration devices are made from ASHRAE filter media or paper products. These paper products are made by a wet-laid technique wherein fibers, e.g. glass or cellulosic fibers, are dispersed in an aqueous binder slurry which is stirred to cause the fibers to become thoroughly and randomly mixed with each other. The fibers are then deposited from the aqueous binder slurry onto a conventional paper making screen or wire as in a Fourdrinier machine or a Rotoformer machine to form a matted paper which includes a binder resin, e.g., a phenolic resin. Pleated filter elements made from such papers can exhibit high efficiencies. However exhibit high pressure drops.
Electrostatically charged synthetic filter media are also used in these filtering applications, and these can attain very high filtration versus pressure drop performance characteristics, at least in their initial charge state. Electrostatically enhanced air filter media and media manufactured by the wet laid process, more specifically with the use of glass fibers, currently have limitations. Electrostatically treated meltblown filter media, as described in U.S. Pat. Nos. 4,874,659 and 4,178,157, perform well initially, but quickly lose filtration efficiency in use due to dust loading as the media begin to capture particles and the electrostatic charge thus becomes insulated. In addition, as the effective capture of particulates is based on the electrical charge, the performance of such filters is greatly influenced by air humidity, causing charge dissipation.
Filtration media utilizing microglass fibers and blends containing microglass fibers typically contain small diameter glass fibers arranged in either a woven or nonwoven structure, having substantial resistance to chemical attack and relatively small porosity. Such glass fiber media are disclosed in the following U.S. patents: Smith et al., U.S. Pat. No. 2,797,163; Waggoner, U.S. Pat. No. 3,228,825; Raczek, U.S. Pat. No. 3,240,663; Young et al., U.S. Pat. No. 3,249,491; Bodendorf et al., U.S. Pat. No. 3,253,978; Adams, U.S. Pat. No. 3,375,155; and Pews et al., U.S. Pat. No. 3,882,135. Microglass fibers and blends containing microglass fibers are typically relatively brittle when pleated, and produce undesirable yield losses. Broken microglass fibers can also be released into the air by filters containing microglass fibers, creating a potential health hazard if the microglass were to be inhaled.
Nonwoven webs have been disclosed for use in air filtration media. In U.S. Patent Application 2006/0137317(A1) to DuPont is claimed a filtration media consisting of a 2-layer scrim-nanofiber (SN) structure for air filters.
The SN medium gives good flux/barrier properties (i.e. high efficiency and low pressure drop). However, the dust-loading capacity is lower than the desired value in certain industrial HVAC applications when filters are challenged with very small dust particles, which can occur when the HVAC system is designed and constructed to have lower efficiency pre-filters in front of the high-efficiency final filters. In the SN structure, the scrim is typically made of nonwoven webs of fiber diameter of 14 to 30 microns which can pre-filter out particles larger than about 5 microns in size. The remaining particles will reach the thin nanofiber layer and quickly fill up the pores and plug up the filters. As the result, filter resistance increases rapidly and thus shortens filter life. Attempts have been made to increase the dust-loading capacity by increasing the basis weight and thickness of the scrim layer but the results are still unsatisfactory for the more demanding situations.
To further complicate the problem, when the humidity of the incoming air is high, dust loaded on the nanofiber layer of the filter media can pick up moisture and swell. It is widely known that a high percentage of atmospheric aerosol is hydroscopic in nature. This further reduces the remaining pore size and creates additional flow restriction and increased pressure drop across the filters. These spikes in pressure drop can create significant problems to HVAC systems.
U.S. Pat. No. 6,521,321 to Donaldson attempts to increase life-time of air filters by layering at least 6 to 7 coarse and fine fiber webs alternatively in a gradient-structure media (e.g. SNSNSN). The number of layering required makes this approach economically unattractive.
In U.S. Pat. No. 7,125,434 to Millipore Corporation attempts to use a deep gradient-density filter consists of three zones of materials for filtering biopharmaceutical fluids. The filter has a depth of at least 0.5 inch and is designed for liquid filtration. The thickness is prohibitive for pleated air filtration uses.
For purifying air both for air conditioning and ventilation purposes air filtering media and air filters produced to date therefore possess a certain, limited capacity to hold dust. High dust capacity filters are either uneconomical or not suited to this application. After reaching a certain limit, which may be expressed in terms of days of use or differential pressure, the media must be replaced. Dust holding capacity is consequently measured in accordance with the maximum amount of dust, which the air filter is able to accept before a lower limit for a certain quantity of air passing through it, and consequently the end of its service life, is reached.
The aim of media design is to achieve maximum dust storage capacity and accordingly service life under conditions of acceptable filtration efficiency. Since however the efficiency on the one hand and the service life on the other hand correlate negatively with each other, it is only possible to achieve an increase in service life in the case of single homogeneous ply media at the expense of the efficiency, unless the installed filter is simply increased in size. The size is however limited by increases in costs, but more especially also by limited space for installation so that for instance in the case of a pleated panel filter the number of the folds can not be increased to the necessary degree.
As a remedy the pleats in the impregnated paper in panel filters may be covered, for example, on the inlet side with a foam material ply, which is to retain a fraction of the dust or at least reduce the kinetic energy of the particles so that there will be an increase in the service life. This method does however involve substantial disadvantages as regards production technology, since the layer of foam material must be bonded to the pleated panel after production of the panel in a further processing stage, for example using beads of hot-melt adhesive.
For internal combustion engines gradient filters are also employed, which are produced from synthetic fiber and become increasingly denser in the direction of flow through the filter. In this case the coarse particles are separated at the surface and the fines are deposited deeper in the filter. A disadvantage here is that for a given amount of installation space substantially fewer pleats can be incorporated. This however increases the impact or inlet flow velocity with all the disadvantages connected therewith: higher pressure losses in the filter inherently owing to the higher flow velocity and deposit of the required dust quantity on less filter area so that the specific dust storage capacity must in this case be many times higher. Additionally such filter media make necessary a complete change in present day production systems, because sealing off the ends of the pleats is no longer possible using conventional hot-melt technology. In fact, the bellow-like folds are injected directly in an injection molding method in a plastic frame in the case of such media, something which is comparatively involved.
Further present day methods for increasing the service life, for example for air conditioning and ventilation applications are described in the German patent publication 9,218,021.3 (utility model) or also the European patent publication 0 687 195. Here a fine filter layer of meltblown micro-fiber non-woven material, which determines the efficiency, is covered with a coarse filter layer on the inlet side so that the dust holding capacity is boosted. The disadvantage is here that for a pleatable design a third layer is generally necessary, which provides the mechanical strength (more particularly stiffness) so that the pleated structure is self-supporting.
The principle of melt blowing is described by Wente, Van A. in the article “Superfine Thermoplastic Fibers” in Industrial Engineering Chemistry, Vol. 48, pages 1342-1346. In gas or, respectively, air filtration generally meltblown layers serve as high efficiency separating filter layers owing to the fine fibers with a diameter or normally somewhat under 1 μm to 10 μm and owing to the frequently applied electret charge and are for example described in the European patent publication 0 687 195, the German patent publication 9,218,021 (utility model) or the German patent publication 19,618,758, the fine meltblown layer always being employed of the outlet side (as a second filter layer). The support materials on the inlet flow side serve as dust storage means in the sense of depth filtration, the meltblown layer serves as a second filter stage in the sense of a fine dust filter. If a dust test is performed with the inlet flow on the “wrong side” that is to say with the meltblown side upstream, the initial degree of separation will be more or less identical, but the dust particle storage capacity goes down, i.e. an undesired filter cake is formed which increases the pressure loss on the inlet side with the meltblown layer surface.
The German patent publication 4,443,158 describes such a structure with the meltblown layer of the inlet side, the extremely high separating power of the meltblown material leading to a high degree of surface filtration, while the support material performs a purely mechanical function. The meltblown layer here produces an increase in the efficiency, but simultaneously a reduction in service life in comparison with the second layer with extremely open pores.
In U.S. Pat. No. 6,315,805 was disclosed that extremely coarse, open, e.g. fluffy meltblown non-woven material may be produced, which when used on the inlet side of a classical filter paper substantially increases service life, namely by approximately 30 to over 300%, dependent on the particular design. In this respect it is less a question of the meltblown non-woven material performing a true filtering function than of the formation of a filter cake on the inlet side of the paper, which embeds itself in the meltblown layer, assumes a substantially looser structure and hence causes less pressure loss. The '805 patent discloses to select for this purpose a fiber diameter of over 10 μm or even over 15 μm.
There remains a need to provide a relatively low cost, high efficiency filter media for these filtration applications which exhibit relatively high dirt-holding and/or air contaminant capacities and relatively low pressure drops as well as low and medium efficiency filter media which exhibit relatively high dirt-holding capacities and relatively low pressure drops.
One object of the present invention is to provide a filter medium and an air filter with which the dust holding capacity may be increased without any substantial change in the efficiency and without any great increase in the thickness of the filter medium.